Would fuelly.com be more informative if, when entering a fill-up, I could say the mileage was highway, city, or mixed? Then you could get a better sense of the difference between the two, and how your car performs at each. The "mixed" tanks could be factored evenly into both the city and highway categories. Or is this too much to ask?
We've talked about this in the past, and I think any self-reporting on highway vs. city mileage is going to be a guess at best. I'm concerned that including the feature would be frustrating for people who want precision-tracking.
Until we get this added, you could always estimate your highway vs. city driving and include it as a tag: "highway", "city", or "mixed". That might start to give you a sense of the difference. (And a sense of how difficult it is to report this information.) I just have a feeling that any numbers that we provide with an official highway vs. city feature are going to be difficult to pin down and might be more frustrating than useful.
there are too many factors even within the city/hwy categories to really matter. As an example, "city" here in San Francisco means pretty much stopping every 100yards for a light or stop sign and climbing innumerable hills - I am getting 20-22 MPG on a Honda Fit! The same car was getting me 28-30 in typical suburban driving - which for many people is still "city" since there are lights and the average speed is 25 -30 MPH. So what does "city" really mean?